1. Field of the Invention
The present invention relates to piezoelectric resonant components, such as an oscillator, a discriminator, and a filter, and especially relates to a piezoelectric resonant component in which a piezoelectric element vibrating in a longitudinal vibration mode is mounted on a substrate.
2. Description of the Related Art
Conventionally, an electro-conductive support body is fixed to a center section (a node portion) of a lower surface electrode of a piezoelectric element that vibrates in a longitudinal vibration mode. While connecting the support body via an electro-conductive adhesive to a pattern electrode on a substrate, a top surface electrode of the piezoelectric element is connected to a pattern electrode of another substrate via wire bonding. A piezoelectric resonant component having such a structure is proposed in, for example, Japanese unexamined published patent application No. 10-200366. The electro-conductive support body is formed by coating an electro-conductive paste on the electrode of the piezoelectric element so as to have a predetermined thickness and then hardening the paste.
In a case of the piezoelectric resonant component having the structure described above, there are some components that cause disorder in the phase characteristics or the impedance waveform as shown in FIG. 1. The disorder of this waveform is generated because the electric connection with the piezoelectric element is achieved by using the electro-conductive support body and the electro-conductive adhesive, while the piezoelectric element is supported but separated from the substrate. Through the electro-conductive support body and the electro-conductive adhesive, vibration of the piezoelectric element proceeds to a substrate side, such that the piezoelectric element and the substrate cause an integral vibration. FIG. 1A shows a waveform of a single piezoelectric element, and FIG. 1B shows a waveform of the piezoelectric element after being mounted on a substrate.
The reason why the vibration of the piezoelectric element is transmitted to the substrate side is because the electro-conductive support body and the electro-conductive adhesive define a pathway for vibration propagation. Thus, in order to suppress such a vibration propagation of the piezoelectric element, the following two methods can be considered.
The first method involves reducing the attaching area in the length direction of the piezoelectric element of the electro-conductive support body, and suppressing the vibration propagation of the piezoelectric element. However, this method results in a reduction in the support strength of the piezoelectric element such that the electro-conductive support body is easily peeled from the substrate or is removed from the piezoelectric element by an impact or dropping shock. Thus, this first method greatly reduces the reliability of the device.
The second method involves forming an electro-conductive support body of rubber elastic materials, such as a urethane resin. However, in order to provide electroconductivity in the electro-conductive support body, electro-conductive fillers such as Ag are used. If the content of this electro-conductive filler is decreased, electroconductivity is greatly reduced. If the content of the electro-conductive filler is increased instead, the elasticity will be greatly deteriorated.
In order to overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric resonant component including an electro-conductive support body arranged to minimize propagation of vibration of a piezoelectric element to a substrate, while reliably achieving electroconductivity and shock-proof characteristics.
According to a first preferred embodiment of the present invention, a piezoelectric resonant component includes a substrate having a pattern electrode disposed on a top surface thereof, and a piezoelectric element vibrating in a longitudinal vibration mode and having an electrode disposed on the lower surface of the substrate, and an electro-conductive support body disposed on an approximately center section in the length direction of the lower surface electrode of the piezoelectric element, wherein the electro-conductive support body is fixed to the pattern electrode of the substrate by the electro-conductive adhesive, and the electro-conductive support body includes a urethane resin containing an electro-conductive filler which constitutes about 75% to about 85% by weight of the electro-conductive support body.
An approximately center section which constitutes a node portion of the electrode of the piezoelectric element is connected and fixed to the pattern electrode of the substrate through the electro-conductive support body and the electro-conductive adhesive. The electro-conductive support body preferably includes a urethane resin that is preferably made of a rubber elastic material. In order to provide electroconductivity in the urethane resin, electro-conductive fillers such as Ag are included. When decreasing the content of the electro-conductive filler, the electroconductivity will reduce, and when increasing the content of the electro-conductive filler, the vibration becomes easy to propagate causing unwanted vibration.
The inventors of the present invention discovered that, if the content of the electro-conductive filler exceeds about 85% by weight, the deterioration of the electrical property due to the vibration propagation will occur. Moreover, the strength deterioration of the urethane resin will also occur. Thus, the upper limit of the electro-conductive filler content in the electro-conductive support body is approximately 85% by weight.
Moreover, if the content of the electro-conductive filler is low, the rubber elasticity is improved and the property deterioration due to the vibration propagation is improved. However, if the content of the electro-conductive filler is less than about 75% by weight, the problems with the electric conduction property are generated.
Thus, the content of the electro-conductive filler in the electro-conductive support body is preferably about 75% to about 85% by weight. In addition, as an electro-conductive filler, an Ag filler, a Cu filler (or a filler which is formed by plating Ag on Cu), an Au filler, or other suitable fillers may be used.
Although it is also possible to replace a urethane resin with a silicone resin as the material for the electro-conductive support body, since there is almost no hardening contraction in the case of the silicone resin compared with the urethane resin, a lot of electro-conductive filler is needed to maintain electroconductivity, and, moreover, workability is bad when using silicone resin. On the other hand, in the case of a urethane resin, there are no such problems.
Since the piezoelectric element and the substrate are fixed through the electro-conductive support body containing a urethane resin in preferred embodiments of the present invention, even if an impact or external force such as a dropping shock is applied, the impact force is absorbed due to the elasticity of the electro-conductive support body. As a result, the impact force is prevented from affecting or cracking the piezoelectric element. Moreover, since the vibration propagation to the substrate is prevented by the elasticity of the electro-conductive support body from the piezoelectric element even when the attachment area of the electro-conductive support body, the piezoelectric element and the substrate are increased, there is no damage or connection-removal of the piezoelectric element despite the impact force applied thereto.
Although it is also possible to use a similar urethane group electro-conductive adhesive as an electro-conductive adhesive for attaching the electro-conductive support body and the substrate, the adhesive strength is not sufficient. On the other hand, in a case of an epoxy group electro-conductive adhesive, the adhesion strength is large, and moreover, since the rubber elasticity is not needed, it is preferable to use an epoxy group electro-conductive adhesive.
The piezoelectric element of various preferred embodiments of the present invention preferably includes two external electrodes which are arranged so that they connect the internal electrode and the pillar-shaped base in which the piezoelectric layer and the internal electrode are laminated in the length direction of the base alternately at one side surface of the base body.
The piezoelectric element is preferably polarized along the length direction thereof.
In such a laminated piezoelectric element, since the piezoelectric longitudinal effect is used, in which the direction of a polarization, the direction of an electric field, and the vibrating direction are the same, the electromechanical coupling coefficient is increased as compared with the piezoelectric element using the piezoelectric lateral effect in which the direction of polarization, the direction of an electric field, and the vibrating direction differ. Thus, the vibrating strength of the laminated piezoelectric element is strong (DELTA f of a resonator is large), and the laminated piezoelectric element tends to cause a vibrating leak. Therefore, by using the electro-conductive support body of preferred embodiments of the present invention, vibrating leaks are prevented and the device characteristics are greatly improved.
Moreover, in the case of the laminated piezoelectric element, since input/output electrodes can be formed on the whole surface thereof, if the input/output electrodes are connected and fixed to the pattern electrodes of the substrate through the electro-conductive support body, the step of connecting the top surface electrode to the pattern electrode of the substrate with wire bonding or other means can be omitted.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention with reference to the attached drawings.